It is known that, to aid a pilot of an aircraft, for example of a transport plane, to manage the piloting, it is customary for the values of parameters which are representative of the flight, such as the speed of the aircraft, to be presented on a particular indicator or a piloting screen [such as a PFD (“Primary Flight Display”) screen for example].
On a PFD piloting screen there is depicted, in general, in this case, in addition to the speed, a minimum allowable limit value for this speed. This minimum value or speed (which can vary as a function of diverse parameters) corresponds to the maximum allowable angle of incidence, beyond which the aircraft is subject to a stalling phenomenon. When he wishes to engage a maneuver, a pilot can visually check the margin available between the current speed of said aircraft and said limit value. If this margin tends to become zero, he knows that his action on a control stick is at risk of not being able to be carried out correctly by the aircraft (risk of stalling or activation of the protection facilities of the flight control laws).
On certain aircraft, the pilot is furnished with a “head up” viewfinder HUD (“Head Up Display”), on which is displayed the direction of the speed vector of the aircraft. The document U.S. Pat. No. 5,808,563 describes for example such a viewfinder. The use of this viewfinder is especially suited to the take-off and landing phases. However, it exhibits drawbacks during the other phases of flight. For example, when piloting a military aircraft by sight, at low altitude, the speed vector, superimposed on the view of the outside world through the windshield, indicates the point toward which the aircraft is directed, without however being representative of the magnitude of the speed. The pilot is therefore furnished with instrument-based piloting information in the HUD viewfinder so as to maneuver the aircraft correctly under conditions demanding sustained attention to the outside environment (for example: presence of nearby reliefs), without having to avert his gaze from this environment visible through the windshield. He is thus furnished with information depicted on the HUD viewfinder. However, he is not furnished with the information depicted on the PFD piloting screen. Specifically, for reasons on the one hand of clarity and of readability of the information and, on the other hand, of visibility through the windshield, it is not desirable to present all the information on the HUD viewfinder. In particular, the current speed of the aircraft and the limit value of the speed cannot be depicted clearly as on a PFD piloting screen. Yet, when the margin between these two items of speed information is disregarded, if the pilot attempts for example to pitch the nose of the aircraft up (by pulling on the control stick) in order to clear a relief although this margin is already substantially zero, the aircraft will not be able to respond to this action and will not modify its trajectory as desired, thus running the risk of striking said relief.
The object of the present invention is to remedy these drawbacks. It relates to a process for aiding the piloting of an aircraft, making it possible for a pilot of the aircraft to be presented with information which is important for the management of piloting.
For this purpose, said process, according to which a first characteristic sign illustrating a speed vector of the aircraft is presented on a display screen, is noteworthy, according to the invention, in that moreover:                at least one longitudinal margin of maneuver of the aircraft is determined, which is expressed as a load factor and which relates to one of the two maneuvers of pitch-up and of pitch-down of the aircraft; and        at least one second characteristic sign which is associated with said first characteristic sign and which illustrates said longitudinal margin of maneuver is presented on said display screen.        
Thus, by virtue of the invention, the pilot is presented with a longitudinal margin of maneuver which may be defined as the difference between an allowable limit value and the current value of a quantity characteristic of the longitudinal motion of the aircraft. This longitudinal margin of maneuver therefore represents the domain of maneuver accessible to the pilot in order to comply with the operational limitations of the aircraft. The invention is therefore especially well suited to an HUD viewfinder.
It will moreover be noted that, contrary to the information usually displayed on a PFD piloting screen comprising as stated above a speed margin, the longitudinal margin of maneuver in accordance with the present invention is expressed in terms of load factor.
This gives rise to several important advantages:                a first advantage is related to the fact that the load factor is more representative of the dynamic behavior of the aircraft than is a value such as the speed. Thus, under certain flight conditions, a margin expressed in terms of load factor is more representative of the margin of maneuver of the aircraft than a margin expressed in terms of speed;        furthermore, under the assumption that the calculations leading to the establishment of the margin of maneuver call upon approximations consisting in linearizing flight mechanics equations over a domain close to the instantaneous conditions of flight of the aircraft, the errors due to said approximations are lower when the margin of maneuver is calculated in terms of load factor than when it is calculated in terms of speed;        another advantage appearing when using an HUD viewfinder results from the fact that the depiction on the HUD viewfinder is carried out in a plane which is perpendicular to the longitudinal axis of the aircraft. Thus, it is more ergonomic to provide a margin of maneuver expressed as a load factor Nz, hence along the vertical axis which is included in said plane of the HUD viewfinder, rather than a speed margin, which corresponds to information along the longitudinal axis which is perpendicular to the plane of the HUD viewfinder; and        additionally, in the case where the aircraft's flight controls are such that the commands entered by the pilot by means of the control stick are expressed, along the vertical axis, as a load factor Nz, said longitudinal margin of maneuver also has the advantage of being homogeneous with the actions of the pilot on said control stick.        
Advantageously, two longitudinal margins of maneuver are determined which relate respectively to the pitch-up maneuver and to the pitch-down maneuver of the aircraft and two second corresponding characteristic signs are presented on the display screen.
Furthermore, advantageously, said second characteristic sign is presented on the display screen in such a way that the distance between said first and second characteristic signs is proportional to said longitudinal margin of maneuver.
In a preferred embodiment, said second characteristic sign is presented on the display screen only if the longitudinal margin of maneuver is less than a predetermined value. This makes it possible not to overload the display screen when the margin of maneuver is sufficient to be able to maneuver the aircraft with no risk of limitation.
In a particular embodiment, the most constraining margin of maneuver between a first load factor margin and a second margin is determined as longitudinal margin of maneuver.
In this case, firstly, to determine the margin of maneuver relating to the pitch-up maneuver, said second margin advantageously corresponds to an angle of incidence margin Δα.
Preferably, said angle of incidence margin Δα is calculated from the following expression:   Δα  =      1    -          [                        (                      Nz                          Δ              ⁢                                                           ⁢              N              ⁢                                                           ⁢              max                                )                ·                  (                                    αmax              -              α                                      α              -              α0                                )                    ]      in which:                Nz is the load factor;        ΔNmax is the maximum value of margin of maneuver depicted;        α is the angle of incidence;        αmax is the maximum angle of incidence; and        α0 is the zero lift angle of incidence.        
Furthermore, secondly, to determine the margin of maneuver relating to the pitch-down maneuver, said second margin advantageously corresponds to a speed margin ΔV.
Preferably, said speed margin ΔV is calculated from the following expression:       Δ    ⁢                   ⁢    V    =      1    -          [                        Nz          +                      Kp            ⁢                          (                                                V                  ⁢                                                                           ⁢                  max                                -                V                            )                                -                      Kd            ⁡                          (                                                ⅆ                  V                                /                                  ⅆ                  t                                            )                                                Δ          ⁢                                           ⁢          N          ⁢                                           ⁢          max                    ]      in which:                Nz is the load factor;        ΔNmax is the maximum value of margin of maneuver depicted;        V is the speed of the aircraft;        Vmax is the maximum speed of the aircraft;        (dV/dt) is the derivative with respect to time of the speed V; and        Kp and Kd are predetermined parameters.        
The present invention also relates to a device for aiding the piloting of an aircraft, which is able to implement the aforesaid process.
According to the invention, said device of the type comprising:                means of determination for determining a speed vector of the aircraft; and        means of depiction for presenting on a display screen a first characteristic sign illustrating said speed vector of the aircraft,is noteworthy in that it moreover comprises auxiliary means of determination for determining at least one longitudinal margin of maneuver of the aircraft, which is expressed as a load factor and which relates to one of the two maneuvers of pitch-up and of pitch-down of the aircraft, and in that said means of depiction moreover present, on said display screen, at least one second characteristic sign which is associated with said first characteristic sign and which illustrates said longitudinal margin of maneuver.        
Preferably, said auxiliary means of determination comprise:                first means for determining a longitudinal margin of maneuver which relates to a pitch-up maneuver of the aircraft; and        second means for determining a longitudinal margin of maneuver which relates to a pitch-down maneuver of the aircraft.        
The present invention moreover relates to an aircraft, in particular a transport plane, which is equipped with a device for aiding piloting such as that stated above.